1. Field of the Invention
The present invention relates to electronic ballasts, and more particularly to electronic ballasts with dimming control from power line sensing.
2. Description of the Related Art
In supplying power to gas-discharge lamps such as fluorescent lamps or cold cathode fluorescent lamps or compact fluorescent lamps, electronic ballasts are widely adopted to keep the lamp current stable.
FIG. 1 shows the typical architecture of a prior art electronic ballast with dimming function for driving a fluorescent lamp. As shown in FIG. 1, the prior art electronic ballast with dimming function mainly comprises a full bridge rectifier 101, a VCC start-up circuit 102, a ballast control IC 103, an NMOS transistor 104, an NMOS transistor 105 and a voltage divider 106.
In the architecture, the full bridge rectifier 101 is used to rectify an AC line input voltage to generate a main input voltage VIN.
The VCC start-up circuit 102, coupling to the main input voltage VIN, is used to start up the generation of a DC voltage VCC.
The ballast control IC 103 is used to generate a high side driving signal VHS for driving the NMOS transistor 104 and a low side driving signal VLS for driving the NMOS transistor 105 to deliver a current ILMP to the fluorescent lamp, in response to the voltage at the DIM input pin 3.
The NMOS transistor 104 and the NMOS transistor 105 are used for generating a square waveform to a LC resonant network. The LC resonant network then converts the square waveform to a current signal ILMP to drive the lamp.
The voltage divider 106 is coupled to a 110V DIM input to generate a DIM control voltage at the DIM input pin 3 of the ballast control IC 103. The 110V DIM input is an additional port to the electronic ballast. In the prior art, the 110V DIM input is generally coupled to an additional dial switch (wall dimmer) or a remote control means, and users have to operate the additional dial switch or the remote control means other than an existing lamp rocker switch to trigger the electronic ballast to adjust the luminance of the lamp.
Through the setting of the DIM input, the NMOS transistor 104 and the NMOS transistor 105 are periodically switched on-and-off by the high side driving signal VHS and the low side driving signal VLS respectively, and the input power is transformed from the main input voltage VIN to the lamp in the form of a current signal ILMP of which the root-mean-square value is corresponding to the setting of the DIM input.
However, since the setting of the DIM input in the prior art has to be done by manipulating an additional dial switch or a remote control means other than an existing lamp switch, users have to pay more cost for the additional dial switch or remote control means. Besides, the additional dial switch may have to be mounted on the wall wherein the wiring between the dial switch and the ballast is bothersome. As to the remote control means, the communication between the transmitter and the receiver needs power, and if the remote control means runs out of battery, then there is no way to dim the lamp unless the battery is replaced.
Therefore, there is a need to provide a solution capable of reducing the cost and eliminating the need of an additional dial switch or remote control means in implementing an electronic ballast with dimming function.
Seeing this bottleneck, the present invention proposes a novel topology of electronic ballast capable of dimming the fluorescent lamp by adjusting the duty ratio of a fixed-frequency square signal according to the count of switching of a corresponding lamp switch, without the need of any additional dial switch or remote control means.